1. Field of the Invention
This invention relates generally to portable sources of heat and more specifically to sources of heat which are capable of producing temperatures in excess of 250.degree. C.
2. Description of the Prior Art
It is often desirable to have a heat source which is capable of providing temperatures in excess of 250.degree. C. Such devices can be used, for example, in an oven, a hot plate, or a space heater.
Such devices are also particularly advantageous for use with heat shrinkable tubing of the type commonly employed in splicing power and telephone cables. Thus, when two cables are spliced together to form a junction, a piece of heat shrinkable tubing can be placed over the junction and a high temperature heat source can be used to shrink the tubing and thereby seal the junction and form an insulation over the splice. It is particularly desirable that the heat from the heat source be applied evenly to the tubing so that it shrinks uniformly around the splice without burning the tubing. This method of cable splicing is often performed in remote locations and in dangerous environments such as mines and manholes wherein a highly combustible methane gas often collects.
There are many types of heat sources in the prior art but most of these sources can be grouped into three general classes: electrical heat sources, combustion heat sources, and chemical heat sources.
Electrical heat sources typically include a resistance element which provides heat in response to a flow of electrical current through the resistance element. The primary deficiency of these electrical devices is associated with their use of large quantities of electrical power. In order to make such heat sources somewhat portable, this power has typically been provided by electrical storage devices such as batteries. Such batteries have been relatively large and heavy. They typically include an acidic flux, and tend to give off combustible gases which would increase the danger associated with an already explosive environment. The electrical heat sources provide for a relatively high temperature only in close proximity to the limited area of the resistance element. These devices do not provide a particularly uniform heat and therefore would have limited value for use with heat shrinkable tubing.
The combustion heat sources rely upon the ignition of a combustible fluid, such as gasoline, or a gas such as propane. An example of this type device is a propane blow torch which is sometimes used as the heat source in power and telephone cable splicing. A propane blow torch relies upon the burning of the propane gas to produce a flame having a high temperature. For use in cable splicing, this type of heat source has the same deficiency as the electrical heat source -- the heat is very localized in proximity to the flame. When used with heat shrinkable tubing, the tubing tends to shrink nonuniformly over its area. Some operators have developed a high degree of skill in moving the propane flame over the surface of the heat shrinkable tubing to provide for more uniform heat distribution. With such a high, localized temperature the tubing is easily burned; this limits the effectiveness of the sealing and insulating characteristics of the tubing.
As exemplified by the propane blow torch, the combustion heat sources typically produce a flame. Such a heat source would be highly undesirable for use in dangerous environments such as mines and manholes wherein highly combustible gases often collect. In such an environment, any heat source producing a flame could cause a severe explosion.
Chemical heat sources are often desirable in a particular environment since they do not produce a flame. Typical of such heat sources is the flameless cooking apparatus disclosed in U.S. Pat. No. 3,314,413. This patent suggests the combination of aluminum powder, sodium hydroxide flakes, copper sulfate, and sodium chloride to which water can be added to produce an exothermic reaction. Although this flameless source of heat can be relatively portable, it suffers from many deficiencies of its own. For example, when the aluminum powder in the mixture is being consumed by the reaction, free hydrogen is given off. This gas is well known to be extremely combustible. Thus, the use of such a device in a mine or manhole would only increase the combustible nature of the environment. Another by-product of this exothermic reaction is hydrochloric gas which is well known to be extremely toxic and particularly corrosive of metals.
The flameless cooking apparatus will provide a temperature of only approximately 150.degree. C. This makes it useful for boiling water but of less advantage for shrinking tubing. Furthermore, the chemicals forming the flameless cooking device have a solid, block configuration that is not conformable to an irregular surface such as the surface of a piece of tubing.
A further deficiency of the flameless cooking apparatus is that it is not self-contained. In order to fire the apparatus, water must be added to the chemical package. Thus, the package must be provided with means for opening the package for insertion of the water. Since a portion of the water will tend to boil from the package, means must also be provided to permit the escape of steam. The powdered aluminum is highly reactive and almost explosive in steam. Thus, the flameless cooking apparatus is relatively unsafe and its temperature range is somewhat limited for use in cable splicing.